1. Field of the Invention
The present invention concerns the ground contact system of vehicles, in particular road vehicles. It concerns both the guiding of the wheels relative to the vehicle body, and the control of the level of the vehicle body, in particular its height and roll.
2. Background of the Invention
The purpose of the wheel guiding system is to ensure a rigorously controlled position of the wheel plane relative to the chassis or body of a vehicle during vertical movements of the suspension. In general, it is easy to identify a plane of symmetry on a vehicle. This longitudinal and vertical plane will be taken as a basis of reference related to the chassis. Remember that the “wheel plane” is the plane perpendicular to the rotation axis of the wheel and passing through the centre of the ground contact area of a tire or elastic cover fitted on the wheel. It is common to call the vertical displacement of the wheel relative to the chassis the “vertical suspension”. The “camber” or “camber angle” is the angle between the wheel plane and a line perpendicular to the ground. “Steering” is the rotation of the wheel plane about a line perpendicular to the ground and passing through the centre of the contact area of the tire on the ground. Finally, the “roll” or “body roll” is the tilting of the body around a longitudinal axis.
The guiding of the wheel plane directly induces the attitude of the tire on the ground, and thus influences the forces undergone by the tire and whether it is in a more or less favourable position for transmitting forces, in particular transverse forces. Now, it is known that those forces are all-important for the vehicle's behaviour and thus for safety.
As is known, road vehicles as used nowadays undergo a rolling motion when rounding a curve, simply by virtue of their suspension. In effect, the centrifugal force results in a load transfer towards the outside of the curve, whose consequence is to increase the compression of the wheel suspensions on the outside of the curve and relax the suspensions on the inside of the curve, hence the roll. This rolling is prejudicial for the comfort of passengers but also for the road holding of the vehicle, particularly because of the wheel camber variation it causes. The most common way currently used to reduce this roll consists in using one or more so-termed “anti-roll” bars. Unfortunately, an anti-roll bar can only restrict roll, by opposing it with a reaction torque. Owing to its principle, it can only limit the amount of roll and cannot therefore totally prevent the camber variations of the wheels in a direction unfavourable to proper operation of the tires. Moreover, although the roll is reduced, the load transfer still takes place and this unequal load distribution on the tires tends to reduce the overall grip potential of the vehicle.
Patent application EP 0 878 378 discloses a vehicle that comprises a camber control mechanism which, viewed in a vertical plane perpendicular to the longitudinal axis of the vehicle, forms a deformable parallelogram. This vehicle uses a vertical suspension system with a slide located in the wheel. The passenger compartment of the vehicle can also tilt relative to the chassis, so that passengers will not be so much affected by the transverse forces due to the centrifugal force. The system enables rigorous control of the orientation of the wheel plane and body. However, this arrangement is rather bulky.